Methods and apparatus for coupling a prosthesis to a spinal segment

ABSTRACT

A method for coupling a prosthesis to a spinal segment in a patient includes the steps of selecting first and second reference points disposed along the spinal segment and pre-operatively measuring a target distance. The target distance extends between the first and second reference points while the patient is in a preferred posture such as the standing position. A prosthesis is coupled to the spinal segment and the prosthesis is then intra-operatively adjusted in order to set the distance between the first and second reference points based on the target distance.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/037,039 (Attorney Docket No. 41564-703.301), filed Feb. 28,2011, which is a continuation of International Patent Application No.PCT/US2009/055914 (Attorney Docket No. 41564-709.601), filed Sep. 3,2009, which claims priority to U.S. Provisional Application No.61/093,922 (Attorney Docket No. 41564-709.101), filed Sep. 3, 2008, thefull disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to medical methods andapparatus. More particularly, the present invention relates to methodsand apparatus used to couple a prosthesis to a spinal segment and adjustthe prosthesis during orthopedic internal fixation procedures. Thisincludes but is not limited to treatment of patients having back pain orother spinal conditions.

A major source of chronic low back pain is discogenic pain, also knownas internal disc disruption. Patients suffering from discogenic paintend to be young, otherwise healthy individuals who present with painlocalized to the back. Discogenic pain usually occurs at the discslocated at the L4-L5 or L5-S 1 junctions of the spine. Pain tends to beexacerbated when patients put their lumbar spines into flexion (i.e. bysitting or bending forward) and relieved when they put their lumbarspines into extension (i.e. by standing or arching backwards). Flexionand extension are known to change the mechanical loading pattern of alumbar segment. When the segment is in extension, the axial loads borneby the segment are shared by the disc and facet joints (approximately30% of the load is borne by the facet joints). In flexion, the segmentalload is borne almost entirely by the disc. Furthermore, the nucleusshifts posteriorly, changing the loads on the posterior portion of theannulus (which is innervated), likely causing its fibers to be subjectto tension and shear forces. Segmental flexion, then, increases both theloads borne by the disc and causes them to be borne in a more painfulway. Discogenic pain can be quite disabling, and for some patients, candramatically affect their ability to work and otherwise enjoy theirlives.

Pain experienced by patients with discogenic low back pain can bethought of as flexion instability, and is related to flexion instabilitymanifested in other conditions. The most prevalent of these isspondylolisthesis, a spinal condition in which abnormal segmentaltranslation is exacerbated by segmental flexion. The methods and devicesdescribed herein should as such also be useful for these other spinaldisorders or treatments associated with segmental flexion, for which theprevention or control of spinal segmental flexion is desired. Anotherapplication for which the methods and devices described herein may beused is in conjunction with a spinal fusion, in order to restrictmotion, promote healing, and relieve pain post-operatively.Alternatively, the methods and devices described should also be usefulin conjunction with other treatments of the anterior column of thespine, including kyphoplasty, total disc replacement, nucleusaugmentation and annular repair.

Patients with discogenic pain accommodate their syndrome by avoidingpositions such as sitting, which cause their painful segment to go intoflexion, preferring positions such as standing, which maintain theirpainful segment in extension. One approach to reducing discogenic paininvolves the use of a lumbar support pillow often seen attached tooffice chairs. Biomechanically, the attempted effect of the ubiquitouslumbar support pillow is also to maintain the painful lumbar segment inthe less painful extension position.

Current treatment alternatives for patients diagnosed with chronicdiscogenic pain are quite limited. Many patients follow a conservativetreatment path, such as physical therapy, massage, anti-inflammatory andanalgesic medications, muscle relaxants, and epidural steroidinjections, but typically continue to suffer with a significant degreeof pain. Other patients elect to undergo spinal fusion surgery, whichcommonly requires discectomy (removal of the disk) together with fusionof adjacent vertebra. Fusion may or may not also include instrumentationof the affected spinal segment including, for example, pedicle screwsand stabilization rods. Fusion is not lightly recommended for discogenicpain because it is irreversible, costly, associated with high morbidity,and has questionable effectiveness. Despite its drawbacks, however,spinal fusion for discogenic pain remains common due to the lack ofviable alternatives.

An alternative method, that is not commonly used in practice, but hasbeen approved for use by the United States Food and Drug Administration(FDA), is the application of bone cerclage devices which can encirclethe spinous processes or other vertebral elements and thereby create arestraint to motion. Physicians typically apply a tension or elongationto the devices that applies a constant and high force on the anatomy,thereby fixing the segment in one position and allowing effectively nomotion. The lack of motion allowed after the application of such devicesis thought useful to improve the likelihood of fusion performedconcomitantly; if the fusion does not take, these devices will failthrough breakage of the device or of the spinous process to which thedevice is attached. These devices are designed for static applicationsand are not designed to allow for dynamic elastic resistance to flexionacross a range of motion. The purpose of bone cerclage devices and othertechniques described above is to almost completely restrict measurablemotion of the vertebral segment of interest. This loss of motion at agiven segment gives rise to abnormal loading and motion at adjacentsegments, which can lead eventually to adjacent segment morbidity.

An alternative solution that avoids some of the challenges associatedwith cerclage devices involves the use of an elastic structure, such astether structures, coupled to the spinal segment. The elastic structurecan relieve pain by increasing passive resistance to flexion while oftenallowing substantially unrestricted spinal extension. This mimics themechanical effect of postural accommodations that patients already useto provide relief.

Spinal implants using tether structures are currently commerciallyavailable. One such implant couples adjacent vertebrae via theirpedicles. This implant includes spacers, tethers and pedicle screws. Toinstall the implant, selected portions of the disc and vertebrae boneare removed. Implants are then placed to couple two adjacent pedicles oneach side of the spine. The pedicle screws secure the implants in place.The tether is clamped to the pedicle screws with set-screws, and limitsthe extension/flexion movements of the vertebrae of interest. Becausesignificant tissue is removed and because of screw placement into thepedicles, the implant and accompanying surgical methods are highlyinvasive and the implant is often irreversibly implanted. There is alsoan accompanying significant chance of nerve root damage. Additionally,the tip of the set-screw clamps the tethers, and this may result inabrasion of the tethers along with generation of particulate weardebris.

Other implants employing tether structures couple adjacent vertebrae viatheir processes instead. These implants include a tether and a spacer.To install the implant, the supraspinous ligament is temporarily liftedand displaced. The interspinous ligament between the two adjacentvertebrae of interest is then permanently removed and the spacer isinserted in the interspinous interspace. The tether is then wrappedaround the processes of the two adjacent vertebrae, through adjacentinterspinous ligaments, and then mechanically secured in place by thespacer or also by a separate component fastened to the spacer. Thesupraspinous ligament is then restored back to its original position.Such implants and accompanying surgical methods are not withoutdisadvantages. These implants may subject the spinous processes tofrequent, high loads during everyday activities, sometimes causing thespinous processes to break or erode. Furthermore, the spacer may put apatient into segmental kyphosis, potentially leading to long-termclinical problems associated with lack of sagittal balance. The processof securing the tethers is often a very complicated maneuver for asurgeon to perform, making the surgery much more invasive. And, aspreviously mentioned, the removal of the interspinous ligament ispermanent. As such, the application of the device is not reversible.

More recently, less invasive spinal implants have been introduced. Likethe aforementioned implant, these spinal implants are placed over one ormore pairs of spinous processes and provide an elastic restraint to thespreading apart of the spinous processes occurring during flexion.However, extension-limiting spacers are not used and interspinousligaments are not permanently removed. As such, these implants are lessinvasive and may be reversibly implanted. The implants typically includea tether structure and a securing mechanism for the tether. The tethermay be made from a flexible polymeric textile such as woven polyester(PET) or polyethylene (e.g. ultra high molecular weight polyethylene,UHMWPE); multi-strand cable, or other flexible structure. The tether iswrapped around the processes of adjacent vertebrae and then secured bythe securing mechanism. The securing mechanism may involve the indexingof the tether and the strap, e.g., the tether and the securing mechanismincludes discrete interfaces such as teeth, hooks, loops, etc. whichinterlock the two. Highly forceful clamping may also be used to pressand interlock the tether with the securing mechanism. Many knownimplementations clamp a tether with the tip of a set-screw, or thethreaded portion of a fastener. However, the mechanical forces placed onthe spinal implant are unevenly distributed towards the specificportions of the tether and the securing mechanism which interface witheach other. These portions are therefore typically more susceptible toabrasion, wear, or other damage, thus reducing the reliability of thesespinal implants as a whole. Other known methods use a screw or bolt todraw other components together to generate a clamping force. Otherlocking methods include the use of a friction fit and are disclosed ingreater detail below. While these methods may avoid the potentiallydamaging loads, the mechanical complexity of the assembly may beincreased by introducing more subcomponents.

A key to proper implantation of many of the spinous process constraintdevices described above is adjusting the tension or size of the devicewhen wrapped around the spinous processes. If the band is not properlyadjusted, it may be too loose and therefore may disengage from theanatomy, or it may not provide adequate resistance to flexion resultingin failure to alleviate the pain or instability. On the other hand, ifthe band is too tight or too small, the device may provide too muchresistance to flexion and unnecessarily restrict the spinal segment'sability to bend, or effect higher than necessary loads to portions ofthe vertebrae or soft tissues. It is therefore imperative to properlyadjust the size and/or tension of the spinous process device. The deviceideally should have a predetermined and preferred configuration whilethe patient is in a preferred posture (e.g. the standing position) andthe device should also provide a force resistive to flexion of thespinal segment while still allowing significantly unrestricted extensionof the spinal segment. For the aforementioned reasons, it would bedesirable to provide improved methods and apparatus for coupling aprosthesis to a spinal segment and adjusting the prosthesis, especiallyduring orthopedic internal fixation procedures. In particular, suchmethods and apparatuses should be easy to perform and be minimallyinvasive.

2. Description of the Background Art

Patents and published applications of interest include: U.S. Pat. Nos.3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866;5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661;5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737;5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221;5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106;6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099;6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527;6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819;6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475;7,163,558; Published U.S. Patent Application Nos. US 2002/0151978; US2004/0024458; US 2004/0106995; US 2004/0116927; US 2004/0117017; US2004/0127989; US 2004/0172132; US 2004/0243239; US 2005/0033435; US2005/0049708; 2005/0192581; 2005/0216017; US 2006/0069447; US2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096;2008/0009866; 2008/0108993; Published PCT Application Nos. WO 01/28442A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1;WO2004/052246 A1; WO 2004/073532 A1; WO2008/051806; WO2008/051423;WO2008/051801; WO2008/051802; and Published Foreign Application Nos.EP0322334 A1; and FR 2 681 525 A1. The mechanical properties of flexibleconstraints applied to spinal segments are described in Papp et al.(1997) Spine 22:151-155; Dickman et al. (1997) Spine 22:596-604; andGarner et al. (2002) Eur. Spine J. S186-S191; A1Baz et al. (1995) Spine20, No. 11, 1241-1244; Heller, (1997) Arch. Orthopedic and TraumaSurgery, 117, No. 1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng.Part H: J. Eng. Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22No. 16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737;Shepherd et al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) MedicalEng. Phys. 23, No. 2: 135-141; and Voydeville et al (1992) OrthopTraumatol 2:259-264.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to methods and apparatus used to couple aprosthesis to a spinal segment and adjust the prosthesis duringorthopedic internal fixation procedures. This includes but is notlimited to treatment of patients having spinal pain or other spinalconditions.

In a first aspect of the present invention, a method for coupling aprosthesis to a spinal segment in a patient comprises selecting a firstand a second reference point disposed along the spinal segment andpre-operatively measuring a target distance. The target distance extendsbetween the first and the second reference points while the patient isin a preferred posture. The method also includes coupling the prosthesisto the spinal segment and intra-operatively adjusting the prosthesis inorder to set the distance between the first and second reference pointsbased on the target distance.

The first reference point may be disposed on a first vertebra and thesecond reference point may be disposed on a second vertebra or a sacrum.The first reference point also may be disposed on a superior surface ofa first spinous process of a first vertebra and the second referencepoint may be disposed on an inferior surface of a second spinous processof a second vertebra. The first reference point may also be disposed onan inferior surface of a first spinous process of a first vertebra andthe second reference point may be disposed on a superior surface of asecond spinous process of a second vertebra. The first vertebra may bedisposed cranial to the second vertebra. The preferred posture maycomprise the standing position or a pain-free position.

The measuring of the target distance may comprise observing a radiographof the patient taken while the patient is in the preferred posture. Theradiograph may be taken pre-operatively and may comprise a lateral viewof the spinal segment. The prosthesis may comprise a tether structureand the coupling may comprise engaging a first portion of the tetherstructure with a superior spinous process and engaging a second portionof the tether structure with an inferior spinous process or a sacrum.

The adjusting may comprise adjusting the prosthesis so that theprosthesis is in a neutral position when the patient is in the preferredposture and the prosthesis may provide a force resistive to flexion ofthe spinal segment while still allowing significantly unrestrictedextension of the spinal segment. The adjusting may also compriseadjusting the prosthesis while the patient is in a position other thanthe preferred posture or observing calibration markings on theprosthesis. Adjusting may also comprise setting the distance between thefirst and second reference points to the target distance.

The method may further comprise verifying that the distance between thefirst and second reference points substantially matches the targetdistance. Verifying may comprise using a gauge to determine the distancebetween the first and second reference points. The method may alsocomprise re-adjusting the prosthesis until the distance between thefirst and second reference points substantially matches the targetdistance.

The prosthesis may comprise a first compliance element and the methodmay further comprise engaging and locking a first constraining apparatuswith the first compliance element in order to limit extension orcontraction thereof during adjustment of the prosthesis. The firstconstraining apparatus may be disengaged from the first complianceelement so as to allow movement thereof. The prosthesis may alsocomprise a second compliance element and the method may compriseengaging a second constraining apparatus with the second complianceelement in order to limit extension or contraction thereof duringadjustment of the prosthesis. The first and second constrainingapparatus may be coupled so as to facilitate alignment and positioningof the first and the second compliance elements on opposite sides of amidline of the spinal segment. The first constraining apparatus may bemoved relative to the second constraining apparatus along one degree offreedom in order to accommodate spinous processes or mid-line ligaments(e.g. interspinous ligament and superspinous ligament) of varyingthicknesses. A driver or tool may be positioned in a central lumen offirst or the second constraining apparatus thereby concentricallyaligning the driver or the tool with a locking mechanism on thecompliance element. The constraining apparatus may be used to provide acounter torque when the first or the second compliance elements areadjusted or when the first and the second constraining apparatus arereleasably coupled together. The prosthesis may be pre-tensioned to adesired value.

The target distance may define a major axis length, and wherein theadjusting comprises using the target distance to determine a targetprosthesis circumference, and adjusting the prosthesis to the targetcircumference. The major axis length may be correlated with the targetcircumference in a lookup table or with calibration markings on theprosthesis.

The method may also include verifying that the prosthesis circumferencesubstantially matches the target circumference. Verifying the prosthesiscircumference may comprise observing calibration markings on theprosthesis. The prosthesis may be re-adjusted until the prosthesiscircumference substantially matches the target circumference.

The method may also comprise selecting a third and a fourth referencepoint disposed along the spinal segment. The distance between the thirdand fourth reference points may define a minor axis having a minor axislength with the minor axis being transverse to the major axis. Themethod may also include measuring the minor axis length on thepre-operative image in order to determine the target prosthesiscircumference. The target circumference may be sufficient for theprosthesis to form a loop encircling a superior spinous process and aninferior spinous process. The prosthesis may provide a force resistantto flexion beyond a desired posture. The third and fourth referencepoints may be on opposite sides of a spinous process and may be used toestimate the length of the prosthesis required to accommodate spinousprocess width. The third and fourth reference points may be disposed ona single vertebra. The minor axis length may be correlated with thetarget circumference in a lookup table or the minor axis length may becorrelated with calibration markings on the prosthesis. The method mayfurther comprise decompressing a portion of the spinal segment.

In another aspect of the present invention, a system for restrictingflexion of a spinal segment in a patient comprises a tether structureadapted to be coupled with a superior spinous process and an inferiorspinous process or sacrum, and a first compliance element coupled withthe tether structure. The system also includes a first constraining toolreleasably coupled with the compliance element so as to hold thecompliance element in a desired position or to limit motion of thecompliance element to a predetermined range. The tether structure may besubstantially non-distensible and the first constraining tool maycomprise an elongate shaft. The first constraining tool may comprise acradle adapted to releasably hold the first compliance element. Thefirst constraining tool may also comprise a plurality of elongate armsthat form a receptacle for releasably holding the first complianceelement and that constrain elongation of the compliance element. Thefirst tool may hold the first compliance element in a desired tension orapply a compressive force to the first compliance element. Thecompressive force may be variable.

The first constraining tool may not limit extension of the firstcompliance element until the first compliance element has extended apre-determined distance. The first constraining tool may be adjustableso as to vary the desired position, tension or the range. The system mayalso include a second compliance element coupled with the tetherstructure and a second constraining tool. The second tool may bereleasably coupled with the second compliance element so as to hold thesecond compliance element in a desired position or to limit motion ofthe second compliance element to a predetermined range. The first andthe second constraining tools may be releasably and symmetricallycoupled together so as to facilitate alignment and positioning of thefirst and the second compliance elements on opposite sides of a midlineof the spinal segment. The first and the second constraining tools maybe movable relative to one another along one degree of freedom, therebyaccommodating spinous processes or midline soft tissues of varyingthicknesses. The first or the second compliance element may comprise alocking mechanism and at least one of the first or the secondconstraining tools may comprise an elongate shaft having a lumen adaptedto receive and align a driver or other tool concentrically with thelocking mechanism. The first or the second compliance element releasablylocks with the first or the second constraining tool. The first or thesecond tool may also be adapted to provide to provide a counter torquewhen the locking mechanism is actuated.

In another aspect of the present invention, a method for treating lowerback pain in a patient comprises providing instructions to the patientto place the lower back into varying positions of flexion anddetermining a threshold position of the lower back in which the patientdoes not experience lower back pain or where lower back pain is reduced.A first image or a set of images of the patient's lower back in thethreshold position is provided and features of the patient's lower backare measured using the first image or the set of images. A constraintdevice is coupled to the patient's lower back and features of the lowerback are re-measured with the constraint device coupled thereto. There-measured features are compared with the measured features and theconstraint device is adjusted so that the patient's lower back is in aposition below the threshold position based on the comparison ofmeasured and re-measured features. Thus, the lower back pain or lowerback instability is reduced or eliminated.

The step of determining may comprise providing the patient with meansfor indicating when pain is experienced. The means may comprise anactuatable switch. The first image or the set of images may comprise oneof an x-ray, MRI, and a CT scan. The providing step may compriseacquiring the set of images from a single continuous movement of thepatient's lower back between painful and pain-free or reduced painpostures.

The measured features may comprise one of intervertebral disc angle,interspinous process distance, and interpedicular distance. Themeasuring may comprise using calipers or an angle measuring device toquantify the features.

The step of coupling may comprise engaging the constraint device with asuperior spinous process and an inferior spinous process or a sacrum.The constraint device may be adapted to provide a force resistant toflexion of the lower back.

The step of re-measuring may comprise providing a second image or a setof images of the patient's lower back with the constraint device coupledthereto. The second image or the set of images may comprise one of anx-ray, MRI, and a CT scan. Re-measuring may comprise placing one or moreradiopaque markers into engagement with the patient's lower back. Theradiopaque markers may be coupled with a spinous process in thepatient's lower back. The comparing step may comprise visually comparingthe first and the second radiographic images or sets of images.Adjusting may comprise adjusting length or tension of the constraintdevice.

The method may further comprise evaluating presence and shape of spinousprocesses of the lower back for coupling of the constraint devicethereto. Facet joint engagement in the lower back may also be evaluated.Evaluating may comprise measuring linear overlap of articular processesof the facet joint. Adjusting the constraint device may compriseadjusting the constraint device so as to increase facet joint engagementin at least one facet joint in the lower back. Adjusting may alsocomprise adjusting the constraint device so as to prevent hyperextensionor locking of at least one facet joint in the lower back. The method mayfurther comprise manipulating the patient's lower back intraoperativelyso that the lower back is in a position at or below the thresholdposition based on the comparison of measured and re-measured features.The manipulating may comprising manipulating the patient's lower back toform or increase lordosis therein.

In another aspect of the present invention, a method for treatingdegenerative spondylolisthesis in a lower back of a patient comprisesproviding instructions to the patient to place the lower back intovarying positions of flexion and providing a plurality of images of thelower back in the varying positions. A threshold position of the lowerback in which a facet joint of the patient's lower back begins to subluxis determined, and a first image of the patient's lower back while inthe threshold position is then provided. A constraint device is coupledto the patient's lower back, and a second image of the patient's back isprovided intraoperatively after the constraint device has been coupledto the patient's lower back. The first and the second images arecompared and the constraint device is adjusted so that the patient'slower back is in a position below the threshold position based on thecomparison of the first and the second images. Thus, subluxation of thefacet joint is reduced or eliminated.

The step of determining may comprise providing the patient with meansfor indicating when pain is experienced such as an actuatable switch. Aneural decompression may be performed concurrently. The determining mayalso comprise assessing engagement of a facet joint or ability of thefacet joint to resist anterior translation of the cranial vertebra withrespect to the caudal vertebra.

The first image may comprise one of an x-ray, MRI, and CT scan. Thecoupling may comprise engaging the constraint device with a superiorspinous process and an inferior spinous process or a sacrum. Theconstraint device may be adapted to provide a force resistant to flexionof the lower back. The second image may comprise an x-ray, MRI, or a CTscan. One or more radiopaque markers may be placed into engagement withthe patient's lower back. The placement may comprise coupling aradiopaque marker with a spinous process in the patient's lower back.

The comparing step may comprise comparing intervertebral disc angle,interspinous process distance, facet joint engagement, or interpediculardistance between the first and the second images or sets of images. Thecomparing may also comprise using calipers or an angle measuring deviceto quantify lower back features in the first and the second images. Thefirst and second images may be visually compared with one another.

The adjusting step may comprise adjusting length or tension of theconstraint device. The method may also comprise evaluating presence andshape of spinous processes of the lower back for coupling of theconstraint device thereto.

In another aspect of the present invention, a method for treating lowerback pain in a patient comprises manipulating the patient's lower backinto a position where the lower back pain is reduced or eliminated andrecording the position. The position is intraoperatively reproduced inthe patient's lower back and a constraint device is coupled to the lowerback.

The step of manipulating may comprise manually manipulating thepatient's lower back, forming, or increasing lordosis in the patient'slower back. The manipulating may also comprise placing the patient in aframe or a chair with an adjustable lumbar member, or flexing a hip. Theflexing of the hip may comprise directing a force proximally through afemoral head to antevert a pelvis thereby forming or increasing lordosisin the patient's lower back. The knee may be restrained.

The step of recording may comprise providing the patient with means forindicating when pain is experienced such as an actuatable switch. Thestep of reproducing may comprise manually manipulating the patient'slower back into the position or forming or increasing lordosis in thepatient's lower back. The step of coupling may comprise engaging theconstraint device with a superior spinous process and an inferiorspinous process or a sacrum. The constraint device may be adapted toprovide a force resistant to flexion of the lower back.

The method may further comprise providing an image of the patient'slower back in the position. The image may comprise an x-ray, MRI, or aCT scan. The method may also comprise providing an intraoperative imageof the patient's lower back after reproducing the position. Theintraoperative image may comprise one of an x-ray, C-arm fluoroscopy,MRI, or CT scan. A radiopaque marker may be coupled with the patient'slower back, such as with a spinous process. The method may furthercomprise intraoperatively characterizing range of motion, segmentalstability, linear stiffness, or bending stiffness of a segment of thepatient's lower back. The constraint device may be adjusted based on thecharacterization of the patient's lower back. The adjusting step maycomprise adjusting length or tension in the constraint device.Additionally, the characterization of the patient's lower back may becompared with a reference guide and the constraint device may beadjusted based on information provided by the reference guide.

In still another aspect of the present invention, a method for treatingspondylolisthesis in a patient comprises providing instructions to thepatient to place the lower back into varying positions of flexion anddetermining a threshold position of the lower back in which the patientdoes not experience translational instability or wherein translationinstability is reduced. A first image or a set of images of thepatient's lower back is provided while the patient is in the thresholdposition and features of the patient's lower back are measured from thefirst image or the set of images. A constraint device is coupled to thepatient's lower back and features of the patient's lower back arere-measured with the constraint device coupled thereto. The re-measuredfeatures are compared with the measured features, and the constraintdevice is adjusted so that the patient's lower back is in a positionbelow the threshold position based on the comparison of measured andre-measured features, thereby reducing or eliminating the lower backinstability.

These and other embodiments are described in further detail in thefollowing description related to the appended drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram illustrating the lumbar region of thespine.

FIG. 1B a schematic illustration showing a portion of the lumbar regionof the spine taken along a sagittal plane.

FIG. 2 illustrates a spinal implant of the type described in US2005/0216017A1.

FIGS. 3A-3B illustrate additional tissue surrounding the spinousprocesses.

FIGS. 4A-4M show an exemplary method of surgically implanting a spinaldevice.

FIGS. 5A-5B show a lateral view of a lumbar region of the spinehighlighting different reference points.

FIGS. 6A-6B illustrate the reference points in FIGS. 5A-5B in asimplified anterior-posterior view of the lumbar region of the spine.

FIGS. 7A-7B illustrate a simplified anterior-posterior view of a lumbarregion of the spine highlighting a plurality of reference points thatmay be used to estimate prosthesis circumference.

FIGS. 8A-8B illustrate an exemplary embodiment of calibration markingson a spinous process constraint device.

FIGS. 9A-9C illustrate an exemplary embodiment of a constraining tool.

FIGS. 10A-10C illustrate another exemplary embodiment of a constrainingtool.

FIGS. 11A-11C illustrate curvature of a patient's spine in differentpostures.

FIGS. 12A-12B illustrate a spinal motion segment in kyphosis andlordosis.

FIGS. 13A-13B illustrate a spinal motion segment with degenerativespondylolisthesis.

FIGS. 14A-14B illustrate a patient in different postures.

FIGS. 15A-15B illustrate a spinal motion segment in kyphosis andlordosis.

FIGS. 16A-16C illustrate application of a lumbar force to a patient'slower back.

FIG. 17 illustrates measurement of various features a spinal segment.

FIG. 18 illustrates the relationship between intervertebral angle andsegmental bending moment under various conditions.

FIGS. 19A-19B illustrate the segmental contribution to total lumbarmotion versus total lumbar motion under various conditions.

FIG. 20 illustrates an exemplary algorithm for diagnosing and treatinglower back pain.

FIGS. 21A-21C illustrate an exemplary tool for constraining twocompliance elements.

FIG. 22 illustrate an exemplary compliance element.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a schematic diagram illustrating the lumbar region of thespine including the spinous processes (SP), facet joints (FJ), lamina(L), transverse processes (TP), and sacrum (S). FIG. 1B is a schematicillustration showing a portion of the lumbar region of the spine takenalong a sagittal plane and is useful for defining the terms “neutralposition,” “flexion,” and “extension” that are often used in thisdisclosure.

As used herein, “neutral position” refers to the position in which thepatient's spine rests in a relaxed standing position. The “neutralposition” will vary from patient to patient. Usually, such a neutralposition will be characterized by a slight curvature or lordosis of thelumbar spine where the spine has a slight anterior convexity and slightposterior concavity. In some cases, the presence of the constraint ofthe present invention may modify the neutral position, e.g. the devicemay apply an initial force which defines a “new” neutral position havingsome extension of the untreated spine. As such, the use of the term“neutral position” is to be taken in context of the presence or absenceof the device. As used herein, “neutral position of the spinal segment”refers to the position of a spinal segment when the spine is in theneutral position.

Furthermore, as used herein, “flexion” refers to the motion betweenadjacent vertebrae in a spinal segment as the patient bends forward.Referring to FIG. 1B, as a patient bends forward from the neutralposition of the spine, i.e. to the right relative to a curved axis A,the distance between individual vertebrae L on the anterior sidedecreases so that the anterior portion of the intervertebral disks D arecompressed. In contrast, the individual spinous processes SP on theposterior side move apart in the direction indicated by arrow B. Flexionthus refers to the relative movement between adjacent vertebrae as thepatient bends forward from the neutral position illustrated in FIG. 1B.

Additionally, as used herein, “extension” refers to the motion of theindividual vertebrae L as the patient bends backward and the spineextends from the neutral position illustrated in FIG. 1B. As the patientbends backward, the anterior ends of the individual vertebrae will moveapart. The individual spinous processes SP on adjacent vertebrae willmove closer together in a direction opposite to that indicated by arrowB.

FIG. 2 shows a spinal implant of the type described in related U.S.Patent Publication No. 2005/02161017 A1, the entire contents of whichare incorporated herein by reference. As illustrated in FIG. 2, animplant 10 typically comprises a tether structure having an upper strapcomponent 12 and a lower strap component 14 joined by a pair ofcompliance elements 16. The upper strap 12 is shown disposed over thetop of the spinous process SP4 of L4 while the lower strap 14 is shownextending over the bottom of the spinous process SP5 of L5. Thecompliance element 16 will typically include an internal element, suchas a spring or rubber block, which is attached to the straps 12 and 14in such a way that the straps may be “elastically” or “compliantly”pulled apart as the spinous processes SP4 and SP5 move apart duringflexion. In this way, the implant provides an elastic tension on thespinous processes which provides a force that resists flexion. The forceincreases as the processes move further apart. Usually, the strapsthemselves will be essentially non-compliant so that the degree ofelasticity or compliance may be controlled and provided solely by thecompliance elements 16.

FIG. 3A is a side view of the lumbar region of the spine having discs Dseparating the vertebral bodies V. The supraspinous ligament SSL runsalong the posterior portion of the spinous processes SP and theinterspinous ligament ISL and multifidus tendon and muscle M runalongside of and attach to the spinous processes SP. FIG. 3B is aposterior view of FIG. 3A.

FIGS. 4A-4M illustrate an exemplary surgical method of implanting aspinous process constraint such as the embodiment of FIG. 2. One of thefirst steps to surgically implant a spinal implant is to make anincision to access the spinal area of interest. FIG. 4A shows the lumbarregion of back K after an incision I has been made through the patient'sskin. FIG. 4B illustrates the lumbar region of the spine after theincision I has been made through the patient's skin. Multifidus muscleand tendon M have been retracted with retraction tools TR to expose thespinous processes.

After the incision has been made, a piercing tool T having a tapereddistal end may be used to access and pierce the interspinous ligamentISL while avoiding the supra spinous ligament SSL, creating aninterspinous ligament perforation P1 superior of the first spinousprocess SSP of interest. Exemplary embodiments of piercing tool T aredisclosed in U.S. patent application Ser. No. 12/478,953 (AttorneyDocket No. 026398-000610US), the entire contents of which areincorporated herein by reference. This surgical approach is desirablesince it keeps the supra spinous ligament intact and minimizes damage tothe multifidus muscle and tendons and other collateral ligaments. Asshown in FIG. 4C, from the right side of the spine, tool T accesses andpierces the interspinous ligament ISL adjacent of the first spinousprocess SSP of interest. The distal end of tool T is shown in dottedline. Alternatively, tool T may access and pierce the interspinousligament ISL from the left side instead. The distal end of tool T iscoupled with tether 102, parts of which are also shown in dotted line.In addition to accessing and piercing the interspinous ligament ISL,piercing tool T also advances or threads tether 102 through perforationP1. As shown in FIG. 4D, tool T is then removed, leaving tether 102positioned through perforation P1. Multifidus tendon and muscle M is notshown in FIGS. 4C and 4D so that other elements are shown more clearly.

FIG. 4E is a posterior view of a section of the spine after the abovesteps have been performed. Often times, the distal tip TI of tool T isdetachable. As shown in FIG. 4E, after tool T accesses and pierces theinterspinous ligament ISL with distal tip TI, distal tip TI is detachedfrom tool T and is left in place in perforation P1 (shown in dottedline) above the first spinous process SSP of interest. Tether 102 lagsbehind tip TI. In some cases, distal tip TI may fully pierce throughinterspinous ligament ISL. In these cases, distal tip TI has passedthrough the interspinous ligament ISL while a portion of tether 102 isleft in place in perforation P1.

After tip TI or a portion of tether 102 a is left in place inperforation P1, another tool may couple with tip TI and pull tip TI suchthat it drags tether 102 a and compliance element 104 a to itsappropriate position relative to the spine, as shown in FIG. 4F.Compliance element 104 a is coupled to tether 102 a and is used toprovide a force resistive to flexion of spinous processes SP. Complianceelement 104 a includes a fastening mechanism or fastening element 106 aand may further comprise a spring, a tensioning member, a compressionmember, or the like. Related compliance elements are described incommonly owned U.S. patent application Ser. No. 12/106,103 (AttorneyDocket No. 026398-000410US), the entire contents of which areincorporated herein by reference.

The steps of accessing the ISL, piercing the ISL, and threading tether102 through a perforation are then repeated for the opposite, lateralside of the spine for an adjacent spinous process ISP, inferior of thefirst superior spinal process SSP of interest. As shown in FIGS. 4G and4H, tool T accesses the interspinous ligament from the left side of thespinal midline and pierces the interspinous ligament ISL, creating asecond perforation P2 located inferior of a second spinous process ofinterest, labeled as inferior spinous process ISP. One of skill in theart will appreciate that the piercing may also be performed from theopposite direction. As shown in FIG. 4G, the inferior spinous processISP of interest is directly adjacent to and inferior of the firstsuperior spinous process SSP of interest. However, it is entirelypossible to perform the described procedure starting with the inferiorspinous process ISP first instead of the superior spinous process SSP,for example, perforation P2 may be created before perforation P1. It isalso possible that there may be a gap of one or more spinous processesSP between the spinous processes of interest. Multifidus tendon andmuscle M is not shown in FIGS. 4G and 4H for clarity of the other shownelements.

As shown in FIGS. 4H, 4I and 4J, similar to the steps shown inconjunction with the first piercing, tether 102 b is pierced throughperforation P2 and left in place along with distal tip TI of tool T(best seen in FIG. 41). Another tool such as a pair of forceps, is thenused to grasp distal tip TI to pull tether 102 b and compliance element104 b in place relative to the spine, as shown in FIG. 4J. Opposingcompliance elements 104 a and 104 b on opposite sides of spinousprocesses SP are oriented in opposite directions. Each complianceelement 104 a, 104 b is coupled with their respective tether 102 a, 102b and has a respective fastening mechanism or fastening element 106 a,106 b. Fastening mechanism 106 a, 106 b are configured to couple withthe tether 102 a, 102 b of the opposing compliance element 104 a, 104 b.Further details on exemplary embodiments of fastening mechanisms aredisclosed in U.S. patent application Ser. No. 12/479,016 (AttorneyDocket No. 026398-000710US) and U.S. Provisional Patent Application No.61/059,543 (Attorney Docket No. 026398-000800US), the entire contents ofwhich are incorporated herein by reference. For example as shown in FIG.4K, tether 102 a is engaged with compliance element 104 b and is thenreleasably coupled thereto with fastening mechanism 106 b. Similarly,tether 102 b is also engaged with compliance element 104 a and is alsoreleasably coupled thereto with fastening mechanism 106 a. Except fortheir orientation, compliance elements 104 a and 104 b are identical.One of skill in the art will appreciate that the tether may enter andexit the fastening mechanism in a number of different directions andconfigurations, and FIG. 4K merely is one exemplary embodiment.

Fastening mechanism 106 may comprise a driver feature 108. As shown inFIG. 4L, the driver feature is adapted to receive a rotating driver toolRT. The driver feature may be a Phillips head, a slotted flat head, aTorx head, a hex head, or the like. Rotation of tool RT, which may beeither clockwise or counter-clockwise, changes the configuration offastening mechanism 106 so as to lock and secure tether 102 in place.This forms a continuous, multi-component tether structure or constraint110 which couples two spinous processes SP together, as shown in FIG.4M. Compliance elements 104 a, 104 b are used to control flexion betweenspinous processes SP while tethers 102 a, 102 b and respective fasteningmechanisms 106 a, 106 b contribute to coupling the spinous processes SPtogether. Depending on the location of the perforations P1 and P2 andthe lengths of the compliance elements 104 a, 104 b, constraint 110 maycouple more than two spinous processes SP together. In general,compliance elements 104 a, 104 b comprise spring-like elements whichwill elastically elongate as tension is applied through tethers 102 a,102 b in an axis generally parallel to the spine. As the spinousprocesses or spinous process and sacrum move apart during flexion of theconstrained spinal segment, the superior tether 102 a and inferiortether 102 b will also move apart. Compliance elements 104 a, 104 b eachinclude spring-like elements which will elastically resist the spreadingwith a force determined by the mechanical properties of the spring-likeelement. Thus, constraint 110 provides an elastic resistance to flexionof the spinal segment beyond the neutral position. Constraint 110 isoften configured to provide a resistance in the range from 7.5 N/mm to20 N/mm but the resistance may be below 3 N/mm or even below 0.5 N/mm.Constraint 110 may also be adjustable in certain dimensions to allowtightening over the spinous processes or spinous process and sacrum whenthe spinal segment is in a neutral position. Other, related tetherembodiments and joining methods are disclosed in U.S. patent applicationSer. No. 12/106,103 (Attorney Docket No. 026398-000410US), U.S. PatentPublication No. 2008/0009866 (Attorney Docket No. 026398-000140US), U.S.Patent Publication No. 2008/0108993 (Attorney Docket No.026398-000150US), U.S. Provisional Patent Application No. 60/936,897(Attorney Docket No. 026398-000400US), the entire contents of which areincorporated herein by reference.

In order for the spinous process constraint device of FIGS. 4A-4M toeffectively reduce back pain and/or spinal instability, the constraintdevice should be sized to prevent or limit painful or unstablepositions, e.g. the constraint should provide a force resistive toflexion of the spinal segment while still allowing significantlyunrestricted extension of the spinal segment. A physician therefore willrequire methods and apparatus for coupling and adjusting the constraintdevice to a spinal segment.

FIG. 5A illustrates a lateral view of a portion of the lumbar region ofthe spine including lumbar vertebrae L3-L5 having spinous processes SPand discs D disposed between vertebrae. One exemplary method ofdetermining the appropriate size of the constraint device involvesobtaining a pre-operative image, such as a radiograph, of the affectedspinal segment. The radiograph is taken while the patient is in adesired, preferably pain-free posture so that the spinal segment is inthe final desired neutral position for the device. From the radiograph,two reference points are selected and these reference points are used toestimate the size to which the constraint device should be adjusted.Additionally, radiopaque gauges may be used to help determinemagnification and distortion effects in the radiograph. A scale may beused in the radiograph for measuring the distance between referencepoints and the physician may compensate for magnification and/or imagedistortion. In a preferred embodiment, a first reference point A1 isselected on a superior surface of a first spinous process SP coupled toa first vertebra and the second reference point A2 is selected on aninferior surface of a second spinous process coupled to a secondvertebra. The second vertebra is below the first vertebra when thepatient is in the standing position. The length between points A1 and A2define a target distance as indicated by the arrow in FIG. 5A. Thus,once the constraint device is applied to the spinal segment, it may beadjusted intra-operatively until the distance between reference pointsA1 and A2 is returned to the pre-operative target distance. This helpsto ensure that the constraint device is in a neutral position when thepatient is standing but will provide a force resistive to flexion of thespinal segment while still allowing significantly unrestricted extensionof the spinal segment. The target distance may be measured withcalipers, rulers, digital radiographic measurements or other suitablegauges. After the distance between reference points A1 and A2 have beenmeasured and adjusted, a verification step may be performed in order toensure that the proper distance is maintained prior to completing thesurgical procedure. If the distance has changed from the target value,the surgeon may re-adjust the constraint device in order to repositionthe spinous processes so that the distance between the two referencepoints A1, A2 is brought closer to the target distance. This fine tuningand re-adjustment may be repeated as required. FIG. 6A illustrates asimplified anterior-posterior view of the spinal segment seen in FIG.5A. The target distance may also be estimated using ananterior-posterior radiograph instead of, or in conjunction with thelateral radiograph.

In another preferred embodiment, the two reference points may be locatedalong different regions of the spinous processes. For example, FIG. 5Billustrates the same view of the spinal segment as in FIG. 5A, yet here,the first reference point B1 is located along an inferior surface of afirst spinous process coupled to a first vertebra and the secondreference point B2 is on a superior surface of spinous process coupledto a second vertebra. The second vertebra is below the first vertebrawhen the patient is in a standing position. A radiograph or other imageof the spinal segment while the patient is in a preferred posture suchas standing, may be used to obtain the pre-operative length betweenpoints B1 and B2 and this is used to define the target distance asindicated by the arrow in FIG. 5B. This length may be used to controladjustment of the constraint device in a manner similar to thatpreviously described with reference to FIG. 5A. One advantage of usingthe reference points illustrated in FIG. 5B is that it may be easier tomeasure this distance intra-operatively using calipers, rulers, gaugepins or sizing blocks than measuring the distance between referencepoints in FIG. 5A. In FIG. 5B, the distance is an “inside” dimension asopposed to the “outside” dimension in FIG. 5B and thus may be easier tomeasure at a consistent location. Adjustment, verification andre-adjustment may also be performed as previously disclosed above. FIG.6B illustrates a simplified anterior-posterior view of the spinalsegment illustrated in FIG. 5B. The target distance may also beestimated using an anterior-posterior radiograph instead of, or inconjunction with the lateral radiograph. Once the constraint device sizehas been set to the target value, optional further adjustment of thedevice allows a physician to set a desired pre-tension value. In stillother embodiments, the measured distance may be the distance betweensuperior surfaces of both spinous processes.

Another embodiment of a sizing algorithm estimates the circumference ofthe spinous process constraint device from the pre-operative radiographof the affected spinal segment. FIG. 7A illustrates ananterior-posterior view of a portion of the lumbar region of the spine.This includes lumbar vertebrae L3-L5 having spinous processes SP anddiscs D disposed between vertebrae. A spinous process constraint deviceSPD encircles two spinous processes SP. In this example, the two spinousprocesses are adjacent one another, however in other embodiments, one ormore spinous processes may be disposed in between the two spinousprocesses to which the spinous process constraint device is coupled. Afirst pair of reference points may be selected on the pre-operativeradiograph to help estimate the circumference to which the spinousprocess constraint device should be adjusted. In FIG. 7A, a firstreference point C1 is disposed on a superior surface or slightlythereabove of a first spinous process coupled with a first vertebra anda second reference point C2 is disposed on an inferior surface orslightly therebelow of a second spinous process coupled with a secondvertebra. The first vertebra is cranial, or above the second vertebrawhen the patient is in a standing position. The reference points C1 andC2 define a major axis and have a major axis length. The major axislength may be measured from the radiograph using calipers, a ruler orother measuring techniques including those previously discussed above.Additionally, the physician may adjust measurements to account fordistortion or magnification in the radiograph. The major axis is shownby the vertical dotted line extending through C1 and C2 in FIG. 7B. Themajor axis length may also be estimated from a lateral view of theaffected spinal segment. The circumference of the spinous processconstraint device SPD may be estimated as twice the major axis lengthplus a constant which accommodates for the constraint device wrappingaround the apex of the upper and lower spinous processes. The constantmay be measured from the radiograph or obtained from a lookup tablebased on other characteristics of the patient's body (e.g. height, bodytype, etc.).

A second pair of reference points optionally may also be selected on thepre-operative radiograph to further help estimate the adjusted size ofthe spinous process constraint device SPD. In FIG. 7A, reference pointsD1 and D2 are taken on either side of one of the spinous processes SParound which the constraint device SPD is encircled. FIG. 7A shows D1and D2 on either side of the inferior spinous process, but they may alsobe located on either side of the superior spinous process. Referencepoints D1 and D2 define a minor axis transverse to the major axis andhaving a minor axis length. The minor axis is illustrated by the dottedline extending through points D1 and D2 in FIG. 7B. Minor axis lengthmay be similarly measured as the major axis length.

Once major axis length and optional minor axis length have been measuredfrom the pre-operative radiograph or other pre-operative image, thecircumference of the spinous process constraint device may be estimated.The spinous process constraint device circumference may be estimated asa rectangle and thus is calculated as twice the major axis length plustwice the minor axis length. The constraint device circumference mayalso be estimated using other models such as by calculating thecircumference of an oval or ellipse. Furthermore, the major axis lengthand minor axis length may be correlated to constraint devicecircumference and a lookup table may provide the correspondingadjustment size to use. Once the constraint device is implanted aroundthe spinous processes, its size is adjusted until its circumferencematches the calculated value or the value provided by the lookup table.The circumference of the constraint device may be measured directly orcalibration markings on the constraint device may be used to indicateconstraint device size. Once the constraint device size has been set tothe target value, optional further adjustment of the device allows aphysician to set a desired pre-tension value.

FIGS. 8A-8B illustrate an exemplary embodiment of calibration markingsthat may be included on the spinous process constraint device to helpestimate device length or circumference. In FIG. 8A, the tether portion402 of a spinous process constraint device 400 is illustrated. The tip404 of the tether is cut at an angle and opposite end 406 is foldedagainst itself and secured to form an aperture that may be coupled witha compliance element having a locking mechanism (not illustrated). Theangle on the tip allows that end to easily be inserted into a lockingmechanism of this or another spinous process constraint device when twoor more devices are coupled together. Calibration markings 408 may beprinted, etched or otherwise affixed to the tether portion 402 of thedevice. Calibration markings 408 may be spaced apart at a known distancefrom one another or numbers may also be included with the markings tofacilitate reading. The calibration marking may indicate tether lengthor circumference or another parameter that allows the physician toadjust the tether to the target value estimated from the pre-operativestanding radiograph. FIG. 8B is an enlarged view of FIG. 8A.

In addition to estimating device length or circumference, it may also bedesirable to characterize the patient's lower lumbar spine in variouspositions to establish a threshold position where pain is experienced.Once this threshold position is determined, the constraint device may beapplied to the patient's spine and adjusted to help ensure that thepatient's back remains at or below the threshold position. Thus clinicalevaluation of flexion exacerbated pain may be linked with imaging baseddiagnostic techniques and various factors may be quantified in thecharacterization of lower back pain. For example, the amount of flexionthat causes or exacerbates pain or subluxation of facet joints may bemeasured and the ability of the native tissue structures to resistflexion or translation may be determined. The nature and degree of anyinstability may also be evaluated. The presence and shape of spinousprocesses on the sacrum may also be evaluated for coupling with aconstraint device. The shape of spinous processes may also be evaluatedalong with a determination of whether modification of the spinousprocesses is required for receiving the constraint device. Also, thestiffness, size and/or tension of the constraint device used to limitflexion may be estimated in order to best treat a specific patient.

FIGS. 11A-11C illustrate the curvature of a patient's lower lumbar spinewhen the patient is in different postures. In FIG. 11A, the patient 1102is in a neutral, standing position and the lower lumbar spine 1104 has adorsally-concave curvature indicated by dotted line 1106 that isreferred to as lordosis. When the spine flexes, as in a forward bending(FIG. 11B) or a seated posture (FIG. 11C), the lordotic curvature of thelumbar spine flattens out. As the spine continues to flex the curvaturemay shift to anterior concavity, as illustrated by dotted line 1108.This is referred to as kyphosis.

Flexion exacerbated pain is often referred to as mechanical low backpain and involves pain when the spine is in a flexed posture. Flexionexacerbated pain may be associate with degeneration of theintervertebral disc (the degenerative cascade is described in greaterdetail by Kirkaldy-Willis). Prior diagnostic techniques often focusedmore on degenerative disc disease as the basis of the clinicalevaluation, including plain film x-ray analysis of disc height, range ofmotion (ROM) and MRI based (magnetic resonance imaging) grading of discdegeneration (e.g. the Pfirrmann MRI classification system).

FIG. 12A illustrates a spinal segment 1202 having two vertebrae V and adisc D with overlapping facet joints 1208. The spinal segment is oftenpain free in a neutral (lordotic) posture as indicated by dotted line1210. However, the segment may become more painful as the segmentflexes, as illustrated in FIG. 12B. The flexion 1204 changes thecurvature of the spinal segment from lordosis to kyphosis as indicatedby dotted line 1212 resulting in pain 1206. The amount of flexion thatcauses pain may vary from patient to patient.

Other pathologies such as degenerative spondylolisthesis (DS) may beexacerbated by flexion as well. In DS, degeneration of the facet jointsreduces the motion segment's inherent ability to resist sheartranslation. This is exacerbated in flexion as facet joint engagementdecreases. FIG. 13A illustrates a spinal motion segment 1302 having twovertebrae an a disc D. The facet joints 1304 have degenerated and thusdo not overlap as much as healthy facet joints (e.g. as illustrated inFIG. 12A). As the spinal segment 1302 is moved into flexion 1306, thefacet joints 1304 separate from one another and thus are less able toresist shear movement 1308 of one vertebra V relative to the othervertebra V and thus they move, distorting the disc D and causing pain asillustrated in FIG. 13B. Typical diagnostic techniques for degenerativespondylolisthesis focus on neurological symptoms and assessment of grossmechanical instability (e.g. range of motion and intervertebraltranslation).

Plain-film radiographs (x-rays) may be taken with the patient in variouspostures, to determine what posture causes pain or instability. X-raysmay be used to measure intervertebral disc angle, inter-spinous processor pedicular distance for preoperative planning and sizing of anyimplant. For example, a patient may be told to bend forward until painis felt. An X-ray taken in this posture will indicate to the clinicianthe segmental posture that elicits pain. This posture represents athreshold position above which the patient experience pain and belowwhich pain is either reduced or eliminated. FIG. 14A illustrates apatient 1402 bending forward so that the lumbar spinal segment 1404 isin flexion 1406 and FIG. 14B illustrates the patient 1402 standing up sothat the spinal segment 1404 is in the neutral position. The patient mayactuate a button or switch to indicate that the pain has started orstopped. The switch may be integrated with the radiography equipment sothat the image is captured at the pain threshold and other postures.

Because radiographic images of spinous processes can be variable(particularly when cartilaginous tissue is present), radiopaque markersmay be used to provide consistent landmarks/fiducials to measureanatomic parameters. For example, tantalum beads may be implanted intothe spinous processes to enable consistent measurement of the separationof the spinous processes. With the beads providing a consistentreference for measurement, the desired (likely pain-free) posture may bemore reliably reproduced in the operative setting.

In addition to evaluating pain vs. posture, this technique may evaluateother posturally-dependent attributes such as facet-engagement.Engagement of the facet joints decreases with segmental flexion, whichmay exacerbate conditions such as degenerative spondylolisthesis.Radiographs may be used to determine the posture at which the facetjoints begin to sublux and resistance to shear load and translationalmotion is reduced. Then, the techniques described above may be used toapply and adjust the flexion constraint in order to prevent thesepostures. FIG. 15A illustrates a lumbar spinal motion segment 1502having two vertebrae V, a disc D, and facet joints 1504. The spinalsegment 1502 is in flexion 1506 while the spinal segment in FIG. 15B isin the neutral position where the spinal segment has a lordotic curve1508. The distance between spinous processes SPD may be measured alongwith the intervertebral disc angle IVDA from the radiographic images,and these parameters may be used to characterize the threshold position.Anatomical measurements from radiographs in painful (FIG. 15A) andpain-free or reduced pain (FIG. 15B) postures may assist a surgeon incorrectly positioning the patient on the operating table, and applyingthe correct size or tension to a flexion-constraining implant. Theobjective being to implant the constraint such that pain-free motion ispermitted, while the painful or unstable motions are restricted.Measurements of interest may include the distance between spinousprocesses (SPD) and intervertebral disc angle (IVDA).

Plain-film radiographs and resulting measurements may also be correlatedto postural measurements of flexion during a patient's normal activitiesof daily living to determine modes and frequency of motions that causepain. For example, a patient may be fitted with a goniometer thatmeasures spinal flexion, or strain gauges on the skin of the lower back.Measurements from the goniometer or strain gauges can be correlated toradiographic measurements described above to estimate lumbar flexion.The patient wears the device during their normal routines, possibly fora day or a week. The device records lumbar flexion, as well as inputs bythe patient to indicate pain. Data recorded by the device can inform thephysician regarding the mode, frequency and postural dependency of thepatient's pain.

The patient's spine may be manually manipulated by the physician, toeffect a pain-free posture, evaluate segmental instability, orpost-operative effectiveness of treatment. In a clinical, diagnosticsetting, this will typically be done by pushing against the lower lumbarspine to create a lordotic curve (much as a back brace in a car seatworks to support the curvature of the spine). A frame or chair with anadjustable lumbar bolster (such as a plunger) may be used to apply arepeatable manipulation to the spine. Alternatively, hip flexion (viathe seat angle) may be used to manipulate the spinal posture. Thesetechniques may also use a proximally-directed force through the femoralhead and hip to antevert the pelvis (rotate forward), and thus inducethe lordotic curvature. The proximally-directed force through thefemoral head will typically be accomplished by applying a force orrestraint to the knee.

These methods and systems may be used by the physician to assess lumbarpostures which are painful vs. pain-free. As described above, thepatient may actuate a switch to indicate the pain threshold. The switchcould provide a time-stamp for dynamic radiography, or trigger an x-raymachine to capture an image. If the frame or chair is radiolucent, thenradiographic images may further enable the physician to reproduce thepain-free posture intra-operatively and apply the constraint structureso that it will prevent motion into the painful posture. Also asdescribed above, implanted radiopaque markers such as tantalum beads mayprovide consistent reference points for radiographic measurements.

FIG. 16A illustrates a normally painful posture where a patient's 1602spine 1604 is flexed 1606. The patient may be sitting or the patient maybe in any other position where the spine is in flexion. FIG. 16B showshow pain is relieved with application of a force 1610 or support thatrestores lordosis 1608 in the lumbar spine 1604 (similar to the lumbarsupport in car seats). Radiographic images in the manipulated, pain-freeposture may be used as described above to implant a flexion constraintdevice such that a spinal alignment is altered to relieve pain, in apreviously-painful posture.

Diagnostic spinal manipulation, as described above, may be performedmore repeatably with a system to apply consistent postural manipulation.One example is a chair as seen in FIG. 16C. The patient 1602 is sittingin the chair having a seat 1616, a back 1614, and a knee brace 1618. Aplunger 1612 can apply manipulative pressure to the lumbar spine 1604,and the knee brace 1618 helps maintain the patient in a desiredposition. The chair preferably is radiolucent so that radiographs may beobtained and used to implant a flexion constraint as describedpreviously.

An apparatus which may be used for this purpose and operates onprinciples similar to the system illustrated above, is the commerciallyavailable “Nada Chair” (http://www.nadachair.com/). A strap loopedaround the lumbar spine provides lordosis-restoring lumbar support. Theopposite end of the strap is looped around the knees so that it can betensioned and apply forces to both the lumbar spine and femoral head(via the knee). Such an apparatus may be used to apply mechanicalmanipulation to the lumbar spine and determine the postural effect onpain. Other braces or orthoses may similarly be used to diagnoseflexion-exacerbated, postural pain.

Manual or mechanical manipulation may also be used intraoperatively toassess segmental biomechanics and instability. The surgeon may use anadjustable table (such as the Jackson Axis table), or instruments suchas laminar spreaders or the Mekanika Spinal Stiffness Gauge device tomeasure ranges of motion or segmental stability to determine the amountand type of restabilization needed from the flexion constraint. This maybe particularly useful for potentially-destabilizing procedures, such asa decompression, where segmental stability may be assessed before andafter the decompression procedure to understand how the segmentalbiomechanics were affected by the procedure. For example, a surgicalinstrument may measure applied load and displacement of vertebralstructures (typically the spinous processes or laminae) to assess alinear stiffness of the spinal structure (usually in N/mm). With thelinear stiffness and a measurement or estimate of the distance from thesurgical instrument to the segmental center of rotation (COR), thesegmental bending stiffness can be estimated, usually in Newton-metersper degree. This may be calculated as:

${K = {0.001\frac{P}{\Delta \; L}R^{2}\frac{\pi}{180{^\circ}}}},$

-   -   K is segmental bending stiffness (usually in N-m/deg);    -   P is the load applied by the surgical instrument (usually in N);    -   ΔL is the distraction or compression applied by the instrument        (usually in mm);    -   R is the moment-arm, or distance from the instrument to the        segmental center of rotation (usually in mm); and    -   The factor 0.001 is used to accommodate the variable L and R        being in millimeters, while the variable K is in        Newton-meters/degree.

FIG. 17 shows a spinal motion segment having two vertebrae V separatedby a disc D and schematically illustrates these measurements. A surgicalinstrument 1702 is inserted between the spinous processes SP of adjacentvertebrae V and measures the distraction or compression, and the loadapplied to the vertebral structures. The moment arm 1706 is distancefrom the center of rotation 1704 of the motion segment to the surgicalinstrument 1702. With an assessment of segmental bending stiffness, thesurgeon can make decisions about appropriate instrumentation forstabilization (e.g. spinal rods), as well as stiffness and tightness ofthe instrumentation. A template, look-up table, software program orother algorithm may be provided with an implant system to make suchdecisions with these measurements. In one exemplary embodiment, a systemfor providing an elastic resistance to flexion may come in multiplestiffnesses. A table provided for use with the implant systems mayrecommend which stiffness is appropriate for a particular patient basedon intraoperatively-measured parameters.

Dynamic radiography, such as obtained from video fluoroscopy or severalframes of x-ray imaging, may also be used to assess instabilities withmore specificity and resolution. In degenerative spondylolisthesis,dynamic radiography may help to identify the intervertebral angle atwhich the facets become unstable. Quantitative motion analysis of thevertebrae may further identify the nature of flexion instability of aspecific motion segment. For example, as the entire spine moves intoflexion, a greater portion of the motion may occur at a single motionsegment, indicating flexion instability in that segment. Furthermore,the instability may present predominantly within a specific portion ofthe total range of motion. Use of these diagnostic, dynamic radiographictechniques may enable the physician to apply a constraint to flexionwhich allows as much natural motion as possible, while preventingpathologically unstable or painful flexion motions. Similar dynamicradiographic measurements may be used to assess the biomechanicalefficacy of any treatment.

As described above, the patient may use a switch to indicate the painthreshold, possibly as a timestamp on the dynamic radiograph. Thedynamic radiographs may also be used to determine facet joint engagementor subluxation, or intervertebral translation, across the range ofmotion. Implantable, radiopaque markers may provide consistentmeasurement references that enable the surgeon to reproduce a desiredposture intraoperatively. These techniques may then be used to apply theflexion constraint such that undesired postures are restricted.

FIG. 18 graphically illustrates the relationship between intervertebralangle and segmental bending moment in two different situations. Dynamicradiography may help to identify the nature of an instability, such asinstability around the neutral zone (curve A), vs. hypermobility orexcessive total ROM (curve B). The nature of the instability may affectthe application of the constraint device. For example curve A mayrequire the constraint to be implanted more tightly; while curve B mayrequire a stiffer constraint.

FIGS. 19A-19B graphically illustrate the relationship between segmentalcontribution to total lumbar motion and total lumbar motion in twodifferent situations. Dynamic radiographs can show the relationshipbetween different motion segments through the total range of motion.FIG. 19A illustrates five motion segments flexing equally throughout thetotal range of lumbar motion. FIG. 19B illustrates five motion segments,where the L4-L5 segment accounts for the largest share of motion as thespine begins to flex. However, the total range of motion for the fivesegments is the same. Dynamic radiography may detect the early-phaseinstability of L4-L5, whereas conventional x-rays may show simply thatall segments have the same total range of motion. Understanding thenature of the segmental instability allows the physician toappropriately apply the flexion constraining implant.

Any of the diagnostic and treatment techniques described above mayutilize software as part of the process. Software may facilitatemeasurement of the anatomical properties, such as intervertebral discangle, tissue stiffness, strain, or dynamic motion properties. Thesoftware package may use these measurements to calculate the appropriateparameters of the flexion constraint implant, such as the appropriatesize, stiffness or tension. The software may intra- or post-operativelyverify that the constraint is implanted such that it has the intendedbiomechanical effect. An exemplary method is illustrated in FIG. 20where the posture that causes pain or instability is first determined2010 and then anatomical features are measured in the painful andpain-free or reduced pain postures 2020. Appropriate parameters for aflexion constraint are calculated (e.g. stiffness, size, tension, etc.)2030 and the surgeon then implants the constraint 2040. The posturalcorrection may then be verified 2050.

As previously described above with respect to FIGS. 4A-4M, the spinousprocess constraint device often includes one or more compliance elementspositioned on opposite sides of a spinous process, across the spinalsegment midline. The compliance elements act like springs to helpprovide the force resistant to flexion of the spinal segment as thespinous processes move away from one another. Because of the complianceelement, adjusting the device to position the spinous processes at atarget distance from one another or to provide a target prosthesis sizesuch as circumference may be difficult. As the device is tensioned, thecompliance element will elongate and prevent the determination of thereference distance. It would therefore be advantageous to provide aconstraining tool that can prevent elongation of the compliance elementduring adjustment of the constraint device. The constraining tool may beused during the deployment and implantation procedure previouslydescribed. The constraining tool is applied to the compliance element totemporarily restrict its elongation or limit extension to a desiredvalue, during adjustment of the device length or circumference. Once thecompliance element is adjusted, the constraining tool may be removed. Aphysician may then continue to adjust the constraint device in order toestablish a desired pre-tension therein.

FIGS. 9A-9C illustrate an exemplary embodiment of a constraining tool500 adapted to hold the compliance element 514 during adjustment of thespinous process constraint device. In FIG. 9A, a plurality of arms 504,506, 508, 510 extend from an elongate shaft 502 to define a cradle orreceptacle 512 for receiving the compliance element 514. The complianceelement 514 is placed or snap fit in the cradle 512 and the arms 504,506, 508, 510 engage opposite ends of the compliance element 514 andprevent expansion thereof. In other embodiments, the cradle size may bevariable or sized larger than the compliance element in order to allow apre-determined amount of extension. Additionally, the plurality of arms504, 506, 508, 510 are spaced apart sufficiently to allow easy access toadjustment screws or apertures on the compliance element 514. Theconstraining tool 500 may be fabricated from any number of metals suchas titanium, stainless steel or polymers such as ABS that are commonlyused for surgical instruments. FIG. 9B shows a side view of theconstraining tool 500 with a compliance element 514 disposed in thecradle 512. FIG. 9C illustrates a perspective view of FIG. 9B. Arms 504,506, 508, 510 preferably do not interfere with operation of the deviceor affect sizing, for example by interfering with the behavior of theconstraint device or offsetting the constraint device away from thespinous processes.

Another exemplary embodiment of a constraining tool 600 is illustratedin FIGS. 10A-10C. In FIG. 10A, constraining tool 600 includes anelongate shaft 602 and a frame having a plurality of axially extendingarms 604, 606, 608, 610. The plurality of arms 604, 606, 608, 610 definea cradle or receptacle 612 for holding a compliance element 616. Thecradle 612 is sufficiently open to allow easy access to the complianceelement including any adjustment screws and apertures that may beincluded with the compliance element. Additionally, aperture 614 alsoallows access to the locking mechanism of compliance element 616. Thelocking mechanism may comprise a locking roller, details of which aredisclosed in U.S. patent application Ser. No. 12/479,016 (AttorneyDocket No. 026398-000710US), the entire contents of which havepreviously been incorporated herein by reference. The constraining tool600 may be fabricated from any of the materials disclosed above withreference to FIGS. 9A-9C. FIG. 10B illustrates a side view of theconstraining tool 600 with compliance element 616 disposed in the cradle612 and FIG. 10C is a perspective view of FIG. 10B.

In embodiments where the constraint device has two compliance elements,it is advantageous to have two constraining tools that cansimultaneously restrict movement of both the compliance elements duringadjustment. FIGS. 21A-21C illustrate an exemplary embodiment of such atool. The tool includes two constraining tools 2102 a, 2102 b that matetogether. Each tool includes an elongate tubular shaft 2104 a, 2104 bwith a rotatable knob 2106 a, 2106 b near the proximal end. The tubularshaft may be a tapered shaft that is threadably coupled with the knobsuch that rotating the knob advances or retracts the shaft. Thus, whenthe shaft is retracted, the arms 2118 a, 2118 b will compress and closearound the compliance element.

A central lumen 2108 a, 2108 b extends from the proximal end of theshaft to the distal end of the shaft and tools may be positioned in thecentral lumen as will be discussed below. A flanged region 2120 a, 2120b near the proximal end of each shaft includes a pin 2112 a, 2112 b andan aperture 2110 a, 2110 b. The pin of one tool may be positioned in theaperture of the opposite tool thereby releasably coupling the two toolstogether and holding them substantially parallel to one another. Thedistal end of the shaft includes an arm 2114 a, 2114 b extendingradially outward from the shaft and having a slotted region 2116 a, 2116b. The distal end of the shaft also has a second arm 2118 a, 2118 b. Thetwo arms on each tool form a cradle for receiving the compliance elementof the constraint device and restricting expansion thereof duringadjustment. The flanged region 2120 a, 2120 b may be sized toaccommodate different patient anatomies, but in preferred embodiments,the longitudinal axes of the two tools are separated by a distance 2130(best seen in FIG. 21C) adequate to straddle a spinous process orinterspinous/supraspinous ligament complex. This distance may varydepending on the patient, but in preferred embodiments may be 10 mm to25 mm, and more preferably 15 mm to 20 mm wide. Additionally, by usingthe pin-aperture coupling mechanism described above, the two tools stillhave one degree of freedom and can be moved in the medial-lateraldirections. The compliance element is held in the cradle such that thelumen is lined up with adjustment features on the compliance element.For example, FIG. 22 illustrates an exemplary embodiment of a complianceelement 2202 having a helical spring-like body 2204. A pin 2208 on oneend of the compliance element secures the tether structure 2206 of thecompliance device thereto and the opposite end includes a locking screw2212 and an tether adjustment rolling mechanism (not illustrated) in thecompliance element housing 2210. When the locking screw 2212 isloosened, a tool may be inserted into the housing to rotate the rollingmechanism thereby tightening or loosening the tether which passestherethrough. Once the adjustment is completed, the locking screw may betightened to lock the roller in place, fixing tether length or tension.These features may be accessed by passing a tool (e.g. a screw driver,hex driver, etc.) through the lumen of the constraint device shaft,where they will line up concentrically with the locking screw or rollermechanism. Another advantage of the constraint tool is that it willprovide a counter torque during the process of tightening the rollermechanism and the locking screw.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

What is claimed is:
 1. A system for restricting flexion of a spinal segment in a patient, said system comprising: a tether structure comprising one or more straps adapted to be coupled with a superior spinous process and an inferior spinous process or sacrum; a first compliance element coupled with the tether structure and configured to elastically elongate as tension is applied through the tether structure; and a first constraining tool releasably coupled with the compliance element so as to hold the compliance element in a first desired position and allow a first predetermined amount of elastic elongation of the first compliance element as tension is applied to the first compliance element though the tether structure.
 2. A system as in claim 1, wherein the tether structure is substantially non-distensible.
 3. A system as in claim 1, wherein the first constraining tool comprises an elongate shaft.
 4. A system as in claim 1, wherein the first constraining tool comprises a cradle adapted to releasably hold the first compliance element.
 5. A system as in claim 1, wherein the first constraining tool comprises a plurality of elongate arms, the plurality of arms forming a constraint to elongation of the compliance element.
 6. A system as in claim 1, wherein the first tool holds the first compliance element in a desired tension.
 7. A system as in claim 1, wherein the first constraining tool applies a compressive force to the first compliance element.
 8. A system as in claim 7, wherein the compressive force is variable.
 9. A system as in claim 1, wherein the first constraining tool does not limit extension or elongation of the first compliance element until the first compliance element has extended or elongated by the first pre-determined amount.
 10. A system as in claim 1, wherein the first constraining tool is adjustable so as to vary the desired position, tension or the range.
 11. A system as in claim 1, further comprising: a second compliance element coupled with the tether structure and configured to elastically elongate as tension is applied through the tether structure; and a second constraining tool releasably coupled with the second compliance element so as to hold the second compliance element in a second desired position and allow a second predetermined amount of elastic elongation of the second compliance element as tension is applied to the second compliance element through the tether structure, and wherein the first and the second constraining tools are releasably and symmetrically coupled together so as to facilitate alignment and positioning of the first and the second compliance elements on opposite sides of a midline of the spinal segment.
 12. A system as in claim 11, wherein the first and the second constraining tools are movable relative to one another along one degree of freedom, thereby accommodating spinous processes of varying thicknesses.
 13. A system as in claim 11, wherein the first or the second compliance element comprises a locking mechanism, and at least one of the first or the second constraining tools comprise an elongate shaft having a lumen adapted to receive and align a driver or other tool concentrically with the locking mechanism.
 14. A system as in claim 11, wherein the first or the second compliance element releasably locks with the first or the second constraining tool.
 15. A system as in claim 13, wherein the first or the second constraining tool is adapted to provide a counter torque when the locking mechanism is actuated. 